Known utility companies can be committed to keeping the power quality of supplied power at a certain level. The utility companies can also be committed to reimbursing customers who suffer through long power outages. Among other things these commitments increase the demand to monitor higher order current or voltage harmonics than previously specified for network protection and control purposes, as well as to locate the fault location in the electrical network quickly in order to minimize the outage time of the power supply or to prevent a build-up of total outage of the power supply due to occasional, incipient faults.
Known protection and control IEDs (Intelligent Electronic Devices) used for distribution network protection and control can have a moderate sampling frequency, for example, between 1 kHz and 2 kHz. This sampling frequency is suitable in obtaining phasor measurements, which can be calculated from the nominal frequency components of the electrical quantities, such as zero voltage, zero current or phase currents and voltages, of the electrical network. For calculating 50 Hz or 60 Hz components of the measurement signals a higher sampling frequency would not provide any benefits.
However, for monitoring the higher order harmonics a sampling frequency of 1 to 2 kHz, for example, may be too low. This is also the case for the fault location purposes, for example, when the fault location calculation is based on a transient phenomenon occurring at an early stage of the fault. The frequency of the transient, which may be used for earth fault location purposes, for example, can lie between 100 Hz and 800 Hz, e.g., and under the 1 kHz limit frequency defined by the Nyqvist theorem when the sampling frequency is 2 kHz. Therefore the sampling frequency of 2 kHz used at present would theoretically be adequate for transient analysis, but in practice, due to a very short duration of the transient, for example, at only a few milliseconds, the sampling frequency of 2 kHz is not adequate for acquiring enough transient data points for accurate fault location calculation.
In order to measure higher order harmonics or in order to measure a bigger amount of transient data points from the transient occurring at the early stage of the fault, the sampling frequency of the intelligent electronic devices could be increased. This, however, will increase the cost of the intelligent electronic devices, the increase of the costs rising to an unacceptable level, for example, if the intelligent electronic devices usable as such should be replaced by new ones only for providing a higher sampling frequency. Therefore, in order to fulfil the commitments relating for example to the monitoring of the higher order harmonics or to minimize the outage times of the power supply by quicker fault location other ways to provide appropriate measuring data should be provided.